I usually use 2-4 chucks of apple/cherry or hickory. Never had an issue with over smoking. 2 birds usually takes 2ish hours. Sometimes more sometimes less. I would suggest a 1/2 full chimney of unlit and then almost a full chimney of lit. This will get your temp up to the 325-350 range where you want to be for just chicken. As with anything, you have to experiment and see what works best for you. Oh and i have fit 3 birds on the top rack with no issue. If you are gonna cook something on the lower, I would either put the chickens in an aluminum foil pan or on the bottom rack. Hope this helps.
Humphrey's Battle Box, Weber Genesis Silver B, Weber Performer